The result of reaction between a liquid sample and one or more reagent, preferably dry, depends on the quantity of the one or more reagent and the volume of liquid sample. Although any type of liquid sample is implied, serum, plasma and blood (also referred to as whole blood) are samples of particular interest. When blood is allowed to clot and the sample is centrifuged, the yellow liquid that sits on top of the blood clot is called serum. If the blood is collected in a tube containing an anticoagulant, for example heparin, and the sample is centrifuged, the yellow liquid that sits on top of the packed red blood cells is called plasma. The packed cell volume (PCV) or hematocrit determines the percentage of red blood cells (RBCs) in whole blood. Since only the RBCs contain hemoglobin, total hemoglobin is highly correlated with hematocrit, except in cases of for example, macrocytic anemia. Some analyzers measure hematocrit by electrical conductivity, and convert the hematocrit measurement to a total hemoglobin concentration, and some analyzers measure total hemoglobin concentration by spectroscopy, and convert the total hemoglobin concentration to a hematocrit value. Spectroscopic calibration algorithms can be developed to measure both hematocrit and total hemoglobin concentration.
Point-of-care Testing (POCT) is defined as medical diagnostic testing performed outside the clinical laboratory in close proximity to where the patient is receiving care. POCT is typically performed by non-laboratory personnel and the results are used for clinical decision making. For the sake of convenience and rapid turnaround time, blood is the sample of choice. Due to the complexity of blood, certain tests can only be performed on serum or plasma.
POCT has a range of complexity and procedures that vary from manual procedures to automated procedures conducted by portable analyzers. POCT is most efficient when the sample of interest can be applied to or loaded onto a test cartridge, the sample inlet capped, and the remaining steps are performed automatically after the loaded test cartridge is inserted into a slot or receptor of an analyzer. Some blood tests, for example coagulation assays and immunoassays require a fixed volume of sample whereas other tests for example electrolytes, do not require a fixed volume of sample. In the case of electrolytes, sample volume may not be an issue if the electrolyte concentration is estimated by measuring electrical activity in the sample. The present invention relates to a point-of-care testing system with automatic sample volume metering, subsequent to applying an unknown sample volume to a disposable cartridge. Applying an unmetered sample volume to test strips is well known; some test strips contain absorbing sections that can accommodate a known volume of plasma, after the red cells are retained in another section of the test strip near the blood application site. In some cases, the hematocrit affects the plasma flow in test strips, and therefore correction for hematocrit may improve accuracy of the analyte measurement. In some systems, a pipette is used that is designed to dispense a predetermined sample volume, after aspirating excess sample into the pipette.
U.S. Pat. No. 6,750,053 to Opalsky et al and U.S. Pat. No. 7,682,833 to Miller et al disclose devices for rapidly metering samples. U.S. Pat. No. 6,750,053 describes a snap-shut seal and discloses in column 11 lines 16-19: “The volume of the metered fluid sample is the volume of the holding chamber 20 between the orifice (48 in FIG. 5) in the wall of the holding chamber and the capillary stop 22.” U.S. Pat. No. 7,682,833 discloses in column 23 lines 39-43: “The location at which air enters the sample chamber (gasket hole 27) from the bladder, and the capillary stop 25, together define a predetermined volume of the sample chamber. An amount of the sample corresponding to this volume is displaced into the first conduit when paddle 6 is depressed.” In the cases of U.S. Pat. No. 6,750,053 and U.S. Pat. No. 7,682,833, the sample in the sample collection well (illustrated in U.S. Pat. No. 6,750,053 as element 12 in FIG. 3) is wasted.
Sample size is a major consideration for POCT systems, especially when it is desirable to use a small drop of blood obtained by puncturing the skin of a body part; the sample is referred to as a pin-prick sample. With some patients, it is difficult to obtain a small drop of blood, therefore there is a need to avoid any blood wastage. This is particularly true for neonatal testing. The present invention overcomes the disadvantage of sample wastage by providing a novel sample volume metering system.